14.3.23 keystatus

Command: keystatus[--shift] [--ctrl] [--alt]

Return true if the Shift, Control, or Alt modifier keys are held down, as
requested by options. This is useful in scripting, to allow some user
control over behaviour without having to wait for a keypress.

Checking key modifier status is only supported on some platforms. If invoked
without any options, the keystatus command returns true if and
only if checking key modifier status is supported.